The invention relates generally to a method for producing one or more single crystalline diamonds through the Microwave Plasma Chemical Vapor Deposition (MPCVD) process. More specifically, the invention relates to a method for producing high quality one or more single crystalline diamonds through the MPCVD process using reactive species of nitrogen.
A diamond is a crystalline allotrope of carbon. Diamonds are renowned for their superlative physical qualities, especially their hardness and high dispersion of light. These properties make diamonds valued for use in jewelry and a variety of industrial applications. Diamonds are broadly classified into natural and synthetic diamonds on the basis of their origin. Natural diamonds are formed naturally in the earth by prolonged exposure of carbon-bearing materials to high pressure and temperature. Scientists have been able to produce synthetic diamonds in laboratory conditions which have the same chemical composition and physical properties as natural diamonds.
Natural diamonds possess a color that can range from colorless to yellow, orange, red, pink, blue, green, brown, and black. The four ‘Cs’, i.e., cut, color, clarity, and carat size, generally determine the price of a diamond. If these three parameters, i.e., cut, clarity, and carat size, are constant, the color of the diamond plays an important role in the pricing of the diamond. The four ‘Cs’ mentioned above are also used for valuation of synthetic diamonds. Fancy-colored diamonds and clear or colorless diamonds are highly priced, and are extensively used in making jewelry. Even the faintest tinge of yellow considerably reduces the price of colorless diamonds. In natural diamonds, brownish-colored diamonds are the most common and relatively inexpensive. Therefore, an aesthetic and economic incentive exists, to produce synthetic diamonds that are not brown, and are colorless or near colorless.
Synthetic diamonds can be produced using a variety of methods. One such method uses high pressure high temperature (HPHT) to produce synthetic diamonds. A carbon substrate such as graphite is exposed to a pressure exceeding 50 kilobars and a temperature exceeding 1200° C. in the presence of a catalyst metal, such as nickel, cobalt, and iron, to produce a diamond. The diamond thus produced is known as a HPHT-grown diamond. However, it is generally difficult and expensive to produce a large-sized, high-quality colorless single crystalline diamond using the HPHT method. Another method, chemical vapor deposition (CVD), is used to synthesize diamonds from the gas phase at below atmospheric pressures and temperatures above 800° C. A mixture of hydrogen and hydrocarbon gas is activated using a variety of methods, such as thermal (hot filament) or plasma (direct current, radio frequency or microwave) activation, or the use of a combustion flame (oxyacetylene or plasma torches). This dissociates the hydrogen gas into atomic hydrogen, and the hydrocarbon gas into active carbon ions, atoms or CH radicals, which deposit on a substrate to form a diamond. The diamond thus produced is known as a CVD-grown diamond.
The quality and growth rate of CVD-grown diamonds in a MPCVD system is dependent on different parameters such as the methane (hydrocarbon gas) flow rate, the hydrogen flow rate, the nitrogen flow rate, the pressure (Torr) and the temperature of the substrate, the type of substrate used, and the microwave power. Most of the parameters mentioned above are inter-dependent, and a window for growing good quality diamond growth is very narrow. However, nitrogen plays a very important role in influencing the growth rate and morphology of diamonds.
A paper titled ‘Characterisation of High-quality Thick Single-crystal Diamond Grown by CVD with a Low Nitrogen Addition’ by A. Tallaire et al., published in ‘Diamond and Related Materials’, Volume 15, Issue 10, October 2006, Pages 1700-1707, describes that addition of small amounts of nitrogen, 2 to 10 ppm, to the gas phase in a high microwave power MPCVD process results in an increase of diamond growth rates by almost a factor of 3.
A paper titled ‘Identification of Synthetic Diamond Grown using Chemical Vapor Deposition (CVD)’ by Philip M. Martineau et al., published in ‘Gems & Gemology’, Volume 40, No. 1, 2004, Pages 2-25, describes that samples grown with added nitrogen were found to range from faint brown to dark brown (or black in extreme cases). Further, for a given sample thickness, there was a strong correlation between the nitrogen concentration in the process gases and the saturation of the brown color. Still further, Cao et al. have reported that the growth of a diamond film on (100) surface shows the highest growth rate, as compared with that on (111) and (110) surfaces with the addition of 100 ppm nitrogen.
A paper titled ‘The Effect of Nitrogen Addition during High-rate Homoepitaxial Growth of Diamond by Microwave Plasma CVD’ by A. Chayahara et al., published in ‘Diamond and Related Materials’ Volume 13, Issues 11-12, November-December 2004, Pages 1954-1958, describes that nitrogen addition can enhance the growth rate of a diamond by a factor of 2 and can create a macroscopic smooth (100) face avoiding the growth hillocks. Further, it was found that the introduction of nitrogen of up to 2 sccm resulted in an increase in the growth rate, and no significant increase in the growth rate was observed above the flow rate of 2 sccm.
A paper titled ‘High Rate Homoepitaxial Growth of Diamond by Microwave Plasma CVD with Nitrogen Addition’, by Y. Mokuno et al., published in ‘Diamond and Related Materials’ Volume 15, Issues 4-8, April-August 2006, Pages 455-459, describes the effect of nitrogen on diamond films grown at the growth temperature range of 1060° C. to 1250° C. The color became dark for the film grown at a temperature of 1060° C. It was reported that this may be attributed to the incorporation of higher amount of nitrogen or the increasing number of defects at the lower temperature region with high N/C ratio in the gas. However, the total amount of nitrogen in the high rate grown diamond was not directly measured. The amount of nitrogen incorporated in the films increased with the increasing nitrogen flow rate. Further, it was reported that the nitrogen concentrations in the films were 8.9 ppm to 39 ppm, which were comparable with the typical value of the HPHT synthetic Ib diamond.
U.S. Pat. No. 7,399,358 titled ‘Synthesis of large Homoepitaxial Monocrystalline Diamond’ assigned to Rajneesh Bhandari, India, describes a method for producing a large homoepitaxial monocrystalline diamond. The method includes placing at least two substrates in a substrate holder in a CVD chamber. The substrates are positioned in such a manner that the growth faces of the substrates form a wedge. The substrates are exposed to the plasma under such conditions that diamond growth occurs in the wedge between the substrates, to form a large homoepitaxial monocrystalline diamond. Nitrogen may be added in the form of nitrogen gas to produce a yellow or brown-colored diamond growth.
WIPO Application WO/2008/099422 titled ‘Method and Apparatus for Producing Single Crystalline Diamonds’, applicant Rajneesh Bhandari, India, describes a method and an apparatus for producing single crystalline diamonds. Diamond seeds are placed in a substrate holder in a CVD chamber. Metal discs are then positioned in the CVD chamber such that high temperature is generated at low microwave power. A plasma is generated in the CVD chamber. Under certain conditions the diamond seeds are then exposed to the plasma to form single crystalline diamonds. The position of the plasma is manipulated to provide uniform growth conditions at the growth surface of the one or more diamond seeds.
U.S. Pat. No. 6,162,412, titled ‘Chemical Vapor Deposition Method of High Quality Diamond’, assigned to Sumitomo Electric Industries, Ltd., Osaka, Japan, describes a method for producing a diamond that contains nitrogen in a small amount through a CVD process. The decrease of the nitrogen atom content in the diamond is achieved by decreasing the nitrogen atom content in the reaction gas.
However, one of the limitations of growing single crystalline diamonds using the available CVD methods is that there is little control over the browning of the diamond which decreases the economic value of the diamond. Further, the available CVD methods consume a considerable amount of time; therefore, these methods are not economical. It is difficult to produce high-quality colorless diamonds at high growth rates using the available CVD methods. Furthermore, these methods require a large amount of nitrogen to produce diamonds at high growth rates which results in browning of the diamonds due to unavoidable extra nitrogen atoms.
In light of the foregoing discussion, there is a need for a method for growing diamonds using the CVD process, which provides high-quality diamonds at high growth rates without resulting in browning of the diamonds. Further, the method should be less time consuming and more economical.